Tensioners for flexible drive means, such as timing belts, timing chains and the like, are well known in the art. The tensioner includes a rotatable member, typically a roller or pulley, which is biased against the belt to maintain a substantially constant tension in the belt as the engine is operated. The pulley is pivotally mounted to the tensioner at a mounting point which is spaced from the axis about which the pulley rotates to create an eccentric about which the pulley can be moved to tension the belt. The difference between the pivotal mounting point and the axis of rotation for the pulley is typically referred to as the “arm” of the tensioner and a spring or other biasing means biases the pulley towards the belt to tension it.
As the forces applied to the belt by the engine can vary significantly as the engine operates, resulting in significant changes in the tension in the flexible drive member, the biasing force which biases the pulley of the tensioner against the belt must be relatively large. Tensioners thus typically include a spring having a relatively large spring force constant to bias the pulley against the belt.
A consideration in the design of tensioners is the angle, with respect to the arm, at which the belt contacts the pulley. Ideally, the contact force between the belt and the pulley should be orthogonal, or close to orthogonal to the arm to achieve proper tensioning as the effective biasing force created by the spring will vary significantly as the contact force angle approaches an angle where the force is inline with the arm, rather than orthogonal to it.
While the above considerations can be effectively dealt with in the design of the tensioner, it can be difficult to install the resulting tensioner on the engine, as taught by commonly assigned U.S. Pat. No. 5,919,107. Specifically, it can be difficult to mount the tensioner on the engine while positioning the arm of the tensioner to achieve the desired angle between the arm and the contact force between the belt and the pulley and while achieving the necessary preload on biasing spring.
Prior attempts to provide a tensioner which can be installed in a reasonable manner have included commonly assigned U.S. Pat. No. 6,149,542, which provides a tensioner with an operating eccentric and an installation eccentric. The installation eccentric is employed to move the tensioner to a position sufficiently far away from the belt to allow installation and routing of the belt. Once the belt is installed, the tensioner is moved along its installation eccentric to place the tensioner into the operating position after which the tensioner arm can move about the operating eccentric.
Typically, the movement of the tensioner along the installation eccentric during installation requires a two-handed operation on the part of the installer wherein the mounting bolt (or bolts) fastening the tensioner to the engine must be loosened with a first hand while the tensioner is moved through its installation eccentric to the desired operating position with the second hand. Once the desired operating position is obtained, the mounting bolt or bolts are fastened with the first hand while the tensioner is held at the operating position with the second hand. As is apparent, this can be an awkward and/or difficult operation to perform. However, even more problematic is the fact that, as the mounting bolt or bolts of tensioner must be loosened during the setting of the operating position, the tensioner can be tilted with respect to the face of the engine. Any such tilt will result in tensioner being set at an improper operating position which will not be apparent until the tensioner mounting bolt or bolts are tightened. In such a case, the installer must re-perform the installation operation until a correct operating position is obtained.
Prior art attempts to avoid installation difficulties have included tensioners with relatively expensive components such as one way clutch mechanisms in single eccentric long-arm tensioners.
It is desired to have a tensioner which is relatively easy to install and which avoids the need for relatively expensive components.